1. Field of the Invention
Embodiments are directed to an imaging system, more particularly to an imaging system having improved image quality and associated methods.
2. Description of Related Art
Recently, image capturing devices have become widely used in portable and non-portable devices such as cameras, mobile phones, webcams and notebooks. These image capturing devices conventionally include an electronic image detector such as a CCD or CMOS sensor, a lens system for projecting an object in a field of view (FOV) onto the detector and electronic circuitry for receiving, processing, and storing electronic data provided by the detector. Resolution and optical zoom are two important performance parameters of such image capturing devices.
Resolution of an image capturing device is the minimum distance two point sources in an object plane can have such that the image capturing device is able to distinguish these point sources. Resolution depends on the fact that, due to diffraction and aberrations, each optical system projects a point source not as a point but a disc of predetermined width and having a certain light intensity distribution. The response of an optical system to a point light source is known as point spread function (PSF). The overall resolution of an image capturing device mainly depends on the smaller one of the optical resolution of the optical projection system and the resolution of the detector.
Herein, the optical resolution of an optical projection system shall be defined as the full width at half maximum (FWHM) of its PSF. In other words, the peak values of the light intensity distribution of a projection of two point light sources must be spaced at least by the FWHM of the PSF in order for the image capturing device to be able to distinguish the two point light sources. However, the resolution could also be defined as a different value depending on the PSF, e.g. 70% of the width at half maximum. This definition of the optical resolution might depend on the sensitivity of the detector and the evaluation of the signals received from the detector.
The resolution of the detector is defined herein as the pitch, i.e., distance middle to middle of two adjacent sensor pixels of the detector.
Optical zoom signifies the capability of the image capturing device to capture a part of the FOV of an original image with better resolution compared with a non-zoomed image. Herein, it is assumed that in conventional image capturing devices the overall resolution is usually limited by the resolution of the detector, i.e. that the FWHM of the PSF can be smaller than the distance between two neighboring sensor pixels.
Accordingly, the resolution of the image capturing device may be increased by selecting a partial field of view and increasing the magnification of the optical projection system for this partial field of view. For example, ×2 optical zoom refers to a situation where all sensor pixels of the image detector capture half of the image, in each dimension, compared with that of ×1 zoom.
Digital still cameras (DSCs) typically employ several groups of lens elements that are mechanically shifted relative to one another in order to create a varying focal length for the whole optical system. In most common multi-use devices having cameras incorporated therein, e.g., mobile phones, notebook computer, web cameras, etc., the optical system is a fixed-focus system, i.e. there are no moving parts. Thus, it is not possible to dynamically change the system's focal length. The most widespread zoom solution offered in camera phones is “a digital zoom,” a solution based on cropping the image down to a smaller size and interpolating the cropped image to the original size, where the missing information is completed in various ways. This solution only emulates the effect of a longer focal length and, by definition, adds no new information to the image.
Thus, use of digital zoom often results in an obvious loss of detail in the zoomed-in image compared to an optical zoom system. As used herein, “digital zoom” refers to signal interpolation where no additional information is actually provided, whereas “optical zoom” refers to magnification of the projected partial image, providing more information and better resolution.
In high-end devices, a mechanical zoom mechanism, similar to the zoom mechanisms in DSCs, may be employed. These systems may incorporate a small motor, typically based on piezoelectric plates, which enable movement of a lens in the Z direction (along the optical axis) and thus create a varying focal length. Such a motor may be around 3 mm in diameter and more than 15 mm in length. Further, predicted cost for mass production is very high compared with fixed-focus lens modules. Additionally, although a mechanical zoom solution provides good quality images, the presence of moving parts results in a much greater vulnerability to physical damage and erosion.
Another technology for achieving optical zoom is a liquid zoom lens. Here, the focal length of a lens changes when a pressure or electro-static force is applied on the liquid inside the lens. However, liquid zoom lens technology suffers from several known problems. For example, changing the lens's focal length in order to achieve zoom also affects the focus, i.e., a system of at least two liquid lenses (one for zoom and one for focus correction) is required. It is therefore hard to implement control mechanisms that enables continuous zoom. Liquid lenses are also known to suffer from pincushion distortion as well as chromatic aberrations. Further, the whole system is characterized by low durability and deteriorating performance over time due to mechanical fatigue.